This reaction supposedly passes through an epoxide intermediate. Can someone please explain why this is necessary?
As I mentioned above, during an isotope labelled mechanistic study, the formation of epoxide intermediate is suggested to explain the scramble isotopic carbon (between carbonyl carbon and neighboring carbon). The detailed arrow pushing mechanism is given bellow:
These steps are subjected to change according to many factors including the reaction conditions and nature of the diol used as correctly pointed out by Zhe and aforementioned MSU website in your comments, which states:
Many factors must be considered when analyzing the course of a pinacol rearrangement. These include:
• Which hydroxyl group is lost as water? or Which intermediate carbocation is more stable?
• What is the inherent shifting tendency (migratory aptitude) of different substituent groups? • What is the influence of steric hindrance and other strain factors on the rearrangement? • Are epoxides formed as intermediates in the pinacol rearrangement?
• Does product stability govern the outcome of competing rearrangements?
• Do the reaction conditions (i.e. type of acid, concentration, solvent and temperature) influence the course of rearrangement?
A reference for the rearrangement through an epoxy intermediate:
C. A. Bunton, T. Hadwick, D. R. Llewellyn, Y. Pocker, Tracer studies in alcohols. Part III. Intermediates in the pinacol–pinacone rearrangement, J. Chem. Soc., 1958, 403-408 [DOI: 10.1039/JR9580000403] (http://pubs.rsc.org/en/Content/ArticleLanding/1958/JR/JR9580000403#!divAbstract).